1. Field of the Invention
The present invention relates generally to a magnetostriction torque sensor for detecting a torsional stress applied to an axle to be measured and more particularly to a magnetic head supporting structure of a magnetostriction torque sensor by which magnetostrictive components of a magnetic field caused by the torsional stress can be detected by a sensing coil or by a Hall effect element.
2. Description of the Prior Art
A torque applied to an axle to be measured can be detected by various well-known methods. For instance, a proximity microswitch is disposed near an axle so as to be actuated when torsional angle of an axle exceeds a predetermined value. In some cases, a strain gauge is directly attached to an outer peripheral surface of an axle and a change in resistance caused by strain is measured by a bridge circuit or an oscilloscope. Further, in the case of detecting a torque applied to a steering shaft of an automotive vehicle, an elastic member is attached to the steering shaft in such a way that a switch is activated by a torsional displacement of the steering shaft. However, the above-mentioned torque sensors are of contact type, because a sensor is attached directly to a movable axle.
In contrast with this, a magnetostriction torque sensor is well known as non-contact type, which is disclosed in Japan Published examined patent application No. S31-942. In this torque sensor, a large wide U-shaped exciting core is arranged near the outer peripheral surface of an axle made of a magnetostrictive material in order to generate a magnetic flux on the surface of the axle, and a small narrow U-shaped sensing core is arranged also near the outer peripheral surface and under the large exciting core in such a way as to cross at a right angle to each other in order to detect the magnetostrictive components of magnetic field caused by a torsional deformation of the axle to be measured. In such a magnetostriction torque sensor, it is inevitably necessary to accurately maintain a predetermined clearance between the exciting core end and the axle or between the sensing core end and the axle. This is because if the clearance fluctuates, it is impossible to accurately detect a torsional stress applied to an axle to be measured, since the magnetostrictive components to be sensed as a voltage is extremely low. Additionally, the clearance is as small as on the order of several to several tens of micron (10.sup.-6 meter). In order to maintain the clearance between the core ends and the axle surface, conventionally an axle to be measured is housed within an outer tubular member being supported by two bearings attached on either end of the outer tubular member. The two cores are fixed to the inner peripheral surface of the outer tubular member in such a way that a predetermined clearance is maintained between the core end surfaces and the outer peripheral surface of the axle to be measured.
In the prior-art magnetostriction torque sensor as described above, however, there exists a serious problems as follows: The basic problem is to require an extremely high precision in manufacturing and assembling the elements of the torque sensor. In more detail, the essential requirements in manufacturing the magnetostriction torque sensor are as follows: (1) the roundness and the straightness of the axle to be measured should be excellent; (2) the roundness and the straightness of the outer tubular member should also be excellent; (3) the concentricity of the axle and the tubular member should be high; (4) the bearings of the outer tubular member to support the axle should be precise in roundness, concentricity, radial dimension, etc.; (5) a rigid magnetostrictive material should be used for the axle to be measured; and (6) highly precise manufacturing and assembling technique or skills are required. Accordingly, the above-mentioned requirements result in a higher manufacturing cost and a lower productivity.
The prior-art magnetostriction torque sensor disclosed in the Japan Patent (S31-942) will be described in more detail hereinafter with reference to the attached drawings under DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS.